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July 13, 2011 -- Spider bites aren't as common as most people and most doctors think, according to a new analysis.

At the same time, researchers also say poor understanding of truly dangerous spider bites delays treatment when a person really has been bitten by a dangerous spider.

For example, the bite of the brown recluse spider can cause death of a sizeable area of skin (skin necrosis) resulting in a deep, scarring ulcer. Yet even in areas infested with brown recluse spiders, true bites are uncommon.

The analysis is published in the July 14 online issue of The Lancet.

"The treatment of patients with suspected spider bite is not straightforward because of overdiagnosis of skin necrosis as being attributable to spider bites while, at the same time, serious [spider bites] ... are not being recognized and treatment is delayed," write Geoffrey Isbister, MD, of the University of Newcastle, Australia, and Hui Wen Fan, PhD, of Butantan Institute, Sao Paulo, Brazil.

Isbister and Fan note that there are more than 41,000 known species of spider, but that very few have bites harmful to humans.

The names of the two U.S. spiders with harmful bites are well known: the black widow and the brown recluse. The two spiders have different poisons:

The bite of the black widow may not be very painful at first. Pain onset usually is gradual and usually takes the form of back and belly pain that can last for hours or even days.

The bite of the brown recluse (and 12 other recluse spiders in North America) can be mild and just cause a mild, itchy bump. But in severe cases, the bite may be painless at first, but over the next two to eight hours develop a sharp, deep pain followed by a burning feeling. The area around the bite reddens and spreads into a deep ulcer that can be as wide as 16 inches across and can take six to eight weeks to heal.

What about other spiders?

"Many additional spiders ... have been implicated," wrote Mayo Clinic dermatologist David L. Swanson, MD, and University of California spider expert Richard S. Vetter, MS, in a 2006 report. "Unfortunately, most of the implicated spiders have been falsely elevated to a status of medical significance through circumstantial implication and repetitive citation in the medical literature."

Spider Bites Rare

Because of the range of symptoms, people often mistake insect bites, chemical burns, allergic reactions, and skin infections for spider bites. But true spider bites are relatively rare, even in households found to be infested with hundreds of brown recluse spiders.

Unless the brown recluse spider is caught in the act, doctors must be willing to challenge the diagnosis of brown recluse spider bite, Swanson and Vetter note.

It's a different story in South America, where recluse spider bites are a major health problem. But in the U.S., the spider's range is from the southern Midwest to the Gulf of Mexico. They've been found from southeastern Nebraska through southern Iowa, Illinois, and Indiana to southwestern Ohio, extending south from northwestern Georgia to central Texas. Unless transported, they are not found west of the Rockies.

Extensive searches turn up no brown recluse spiders in the southern third of Georgia and in Florida. But that doesn't stop people from thinking they've been bitten by a brown recluse. No recluse species are native to Florida, yet over a six-year period Florida poison control centers received 844 brown recluse bite reports -- 15% of them from medical personnel.

In Georgia, researchers asked people to send them their brown recluse spiders. Some 1,060 people sent in spiders they believed to be brown recluses. Only 19 really were. Most turned out to be harmless Southern house spiders, wolf spiders, or orb weavers. The experts who conducted the study went on a field trip to seek out brown recluse spiders in Georgia; they found none. Yet over that period there were hundreds of reported brown recluse bites.

Brown recluse spiders are not rare in many areas of the U.S. In a 2009 report, Vetter and colleagues noted that a family in Lenexa, Kan., collected 2,055 brown recluse spiders over six months in their home. In an Oklahoma barn, spider hunters found 1,150 brown recluses in just three nights.

Even though the brown recluse is a common house spider in several states, bites are relatively uncommon. For example, the four family members in the Lenexa, Kan., house suffered no bites after eight years in the house.

It's lucky spider bites are rare, as there's no definitively proven treatment. While anti-venom exists, there's no proof it helps -- although there's a long history of safe use, so Isbister and Fan recommend continuing to use it. However, anti-venom probably does not prevent the ulceration and scarring of a severe brown recluse bite.

"Studies are needed to prevent the unnecessary use of ineffective antivenom, which puts patients at risk of allergic reactions," they suggest.